Pye P78 - Silvered Scale lamps and Unknown component markings

[Barnjet]

Originally Posted by 60 oldjohn

47 ohms ? is that for the 200v - 225v setting?

For the 226v - 250v setting it should be 57 ohms. Todays (UK setting)

47 ohms measured across the 2 arrowed terminals in the phot. When taken from the blue wire terminal (the 200-225 tap) it gives 41.2 ohms

[Barnjet]

Originally Posted by cathoderay57

Hi Max, the heater resistances are, as said by others, reasonable. What I was getting at is that sometimes the heater develops a resistive path to the cathode, which can cause problems. Therefore I meant you should measure resistance between one of the heater pins and the cathode. Cheers, Jerry

My apologies Jerry, I got the wrong end of the stick there. If I had considered your post a bit more critically, then it would be ridiculous for the cold heater filaments to have megohm or higher resistances!

In any case, all cathode to heater resistances checked and all OC, which is good.

Max

[Barnjet]

Some shiny new 6.5V scale lamps installed. 3-core mains lead fitted. Replaced waxy cap C22 (AF coupling on V3b control grid). Lamp limiter set to "lamp".... stand back.... switch on...........

.........Norwegian parrot!!

Nothing! Not even the scale lamps.

Continuity checked from mains supply onwards... all good until rotary power switch. For some reason it had failed. Bridged the switch terminals and used the mains socket switch to power up. Bingo! Scale lamps on, valve heaters glowing and a little later R5, then R4 received loud and clear.

With limiter by-passed, checked all pin voltages and pretty close to specs, except V2 and V3 cathode voltages which measured 3.9V (spec = 8.0) Not sure if this a problem?

Checked current draw = 141mA which at 235V yields 33 watts. Trader sheet states 25W equating to 106mA, so went looking for causes. Replaced another waxy cap C19 (AF coupling to V3a grid) and this reduced draw to 133mA. This seems pretty close to spec, so is it worth further fault finding?

Also, the set needs to be powered on/off via the mains socket s/w. It strikes me this is not a bad way of operating the set, as it means the set is isolated when not in use. Do members have a view on this?

Thanks

[G6Tanuki]

Originally Posted by Barnjet

Also, the set needs to be powered on/off via the mains socket s/w. It strikes me this is not a bad way of operating the set, as it means the set is isolated when not in use. Do members have a view on this?

Thanks

I prefer to keep things as-designed; a radio with a bypassed on-off switch could be a hazard if used by someone unaware of the bypass, who turned the volume-control down to zero, heard the 'click' and so logically thought it was turned-off, and so went away for the weekend....

[Plenty of us also have un-switched mains-sockets... or run our radios from multi-way power-strips that also power other stuff].

Keeping it original - and intuitive - makes a lot of sense.

[cathoderay57]

The volume control (logarithmic) is an odd value, 800k, but I would have thought you'd get away with a 1M. Bowood stock a 1M log with DP switch but you'd need to check the size to make sure it would fit, and you'd probably need to hacksaw off some of the shaft. https://www.bowood-electronics.co.uk...-log-switched/ Jerry

[Barnjet]

Originally Posted by G6Tanuki

I prefer to keep things as-designed; a radio with a bypassed on-off switch could be a hazard if used by someone unaware of the bypass, who turned the volume-control down to zero, heard the 'click' and so logically thought it was turned-off, and so went away for the weekend....

[Plenty of us also have un-switched mains-sockets... or run our radios from multi-way power-strips that also power other stuff].

Keeping it original - and intuitive - makes a lot of sense.

Thanks, safety and originality - both good points.

[Barnjet]

Originally Posted by cathoderay57

The volume control (logarithmic) is an odd value, 800k, but I would have thought you'd get away with a 1M. Bowood stock a 1M log with DP switch but you'd need to check the size to make sure it would fit, and you'd probably need to hacksaw off some of the shaft. https://www.bowood-electronics.co.uk...-log-switched/ Jerry

Thanks Jerry, very helpful. I would not have known where to find this sort of component. Compatible size is my biggest concern, so I have emailed the supplier to ask for a technical drawing if one exists. Of course, it may be possible to repair the original part, which is what I will try first.

[Barnjet]

A fairly lengthy and very fiddly repair to the power switch, involving endless unsoldering, tab bending, splitting the volume pot from the power switch and drilling out 3 x rivets. I've taken a series of phots which makes the repair pretty self evident. The problem seemed to be the contact studs which needed fine emery paper to remove non-conductive deposits.

The main thing to watch is the cups either end of the spring which have a desire to boing off into oblivion. Refitting them was very tiresome, more cup catapulting, but with the help of some contact adhesive to hold the cups on the spring, I got there.

Switch now working as it should.

I had hoped to simply unsolder the joints and remake them on completion. However, I found it very difficult to do this as the wires were turned back on themselves through the loops on the tabs. All I achieved was to keep resoldering the joint, so eventually I admitted defeat and resorted to snipping the wires. Is there a better way of unsoldering these types of joint?

[cathoderay57]

Well done, Max. A nice neat repair. Re: wire wrapped joints, they can be the bane of your life leading to overheated insulation and components, and broken solder tags. In a current thread, stevehertz mentions the practice of cutting capacitors leaving a short stub of wire on the tag then cleaning and tinning it to solder the new wire onto. If you can do this with a small loop from the new wire or component wrapped around the stub this is mechanically a good solution. Simply holding one wire against another (solder and stick) can be done in extremis but is not as robust as steve's method. Cheers, Jerry

[Barnjet]

Originally Posted by cathoderay57

Well done, Max. A nice neat repair. Re: wire wrapped joints, they can be the bane of your life leading to overheated insulation and components, and broken solder tags. In a current thread, stevehertz mentions the practice of cutting capacitors leaving a short stub of wire on the tag then cleaning and tinning it to solder the new wire onto. If you can do this with a small loop from the new wire or component wrapped around the stub this is mechanically a good solution. Simply holding one wire against another (solder and stick) can be done in extremis but is not as robust as steve's method. Cheers, Jerry

Thanks, Jerry, the effort was worthwhile. I read the Stevehertz thread https://www.vintage-radio.net/forum/...d.php?t=191690
and the method of installing new components is a good solution. I tried this method on a cap (see phot), but I don't see how you can re-apply any insulation. Maybe heatshrink?

Secondly, any tips on how to refurbish this lacquered brass escutcheon behind the knobs? Or perhaps just leave as is?

[cathoderay57]

Hi Max. Re: insulation, on the "closed" end such as a short stub on a valve pin socket, using the old style waxed or cotton braided sleeving you could compress it along the capacitor wire, solder, then release it and with a bit of persuasion it would stretch out again to cover the joint. If there is a "free" end of the component then simply use slightly oversize sleeving of any sort to slide over the soldered joint. If the end stub that you are soldering to is short enough then you don't really need to worry about insulating it, unless the adjacent components are very tightly packed. I've never tackled restoration of lacquered brass; you could remove and spray it a sympathetic colour I suppose. Suffice it to say if you scrape or sand it the end result could look a whole lot worse.... Maybe somebody else has a cunning plan. Cheers, Jerry

[Barnjet]

Hi Jerry, Re: Insulation - good tips, thanks.

Re: Brass work. I'm inclined now to leave well alone. As you say, over enthusiastic use of abrasives would probably make it look a whole lot worse. It is, after all, over 60 years old and it should show its age. Max

[Barnjet]

Buffed up the Bakelite, buttoned up the set and ready for action. Despite the fiddly power switch repair, I found this this a very straightforward project.

I'm impressed with the set design: its small size; relatively low power consumption; good signal reception and the automatic tone control.

Thanks to all for help and advice

Max